Magnetic tunnel junctions (MTJ's), including spin torque transfer (STT) MTJ's, comprise two layers of magnetic material separated by a thin insulator. The magnetization direction of one magnetic layer is fixed and the magnetization direction of the other magnetic layer can be changed by applying a voltage or current to the MTJ. When the magnetization directions of the fixed, and the free layers are opposite or antiparallel, the MTJ has a higher resistance than when the magnetization directions of the fixed and free layers are parallel. The resistance of an MTJ can be measured by applying a small read current thereto. The MTJ can represent a digital “0” in one of these magnetic states and to represent a digital “1” in the other thus allowing the MTJ to be used as a memory element, a magnetic random access memory element (MRAM).
After the resistance of a target MTJ is measured, it must be compared to a reference resistance to determine whether the measured resistance represents a parallel- or antiparallel-state MTJ. Conventionally, a reference resistance is determined by passing a reference current through a parallel-state reference MTJ and by passing the reference current through an antiparallel-state reference MTJ. FIG. 1 illustrates a conventional reference circuit. In FIG. 1, a plurality of parallel-state MTJ's 102 are connected between a first input line 104 and a first output line 106, and a current IRp passing through individual ones of these parallel-state MTJ's 102 is measured. Similarly, a plurality of anti-parallel state MTJ's 108 are connected between a second input line 110 and a second output line 112, and a current IRap passing through individual ones of the antiparallel MTJ's 108 is measured. These two currents IRp and IRap are added, and the sum is divided by two to determine the average current flow through MTJ's in these two states. A reference resistance is calculated from this reference current and an applied voltage.
Due in part to manufacturing differences, the resistances of different MTJ's may vary. FIG. 2 shows a first distribution 202 of resistances of a given group of MTJ's when they are in a magnetically parallel state and a second distribution 204 of this group of MTJ's when they are in a magnetically antiparallel state. The first distribution 202 extends from approximately 2000 ohms to 3500 ohms while the second distribution 204 extends from approximately 4250 ohms to 7250 ohms When a reference resistance is determined from such MTJ's in a conventional manner, that resistance may have a third distribution 206 from about 3000 ohms to 4250 ohms This third distribution 206 thus overlaps with at least the first distribution 202 and has little or no spacing from the second distribution 204. This lack of a clear separation between the resistance distributions may make it difficult to determine with confidence the magnetic state of a target MTJ. It would therefore be desirable to determine a reference resistance in a manner that produces a narrow distribution that is spaced from the distributions of measured resistances of MTJ elements in a parallel or antiparallel state.